A. Field of Invention
This invention relates to a closed-loop hydraulic system design for control of a linear hydraulic actuator using a unique valve circuit and a unique electronic control circuit to allow faster, more efficient, more accurate motion in both position and force loop control.
B. Description of Related Art
A typical closed-loop hydraulic system, in this case a hydraulic cylinder, is shown in FIG. 1, and usually consist of a hydraulic source, 3, with a hydraulic reservoir associated therewith, servovalves, 2, a cylinder, 1, and a position feedback device, 4. The function of the system is for the cylinder to move with great power to a mid-stroke position corresponding to an input command. The input command, typically a voltage signal, is conducted into one half of a comparator circuit in the control electronics, 5. Actual cylinder position, continually measured by the position feedback device, 4, is also conducted by voltage signal into the comparator. The difference between the input and the feedback, assuming there is a difference, is a position error signal that is then amplified by an amplifier, 6, of the control electronics and used to drive the servovalves, 2. Depending upon the error signal, the servovalve directs pressurized oil from the hydraulic source into either the cap or rod end of the cylinder, causing the cylinder to either extend or retract, respectively. Drain oil returns to the reservoir. This process of comparing command and feedback, then driving the cylinder correspondingly continues in a closed-loop fashion until the error is reduced to zero, meaning that the cylinder has necessarily reached its desired position corresponding to the original command. Any movement induced in the cylinder, such as from a varying external load, is met with a corresponding error signal and is similarly driven back to zero by the servo loop.
A closed-loop hydraulic system can be designed that operates on virtually any other mechanical characteristic, including force. The force loop system is similar to the position loop system above except that the position feedback device is replaced with a force feedback device. Any difference between the input command force and the actual force measured by the force feedback device results in an error signal that drives a servovalve. Like the closed loop position control, the cylinder continues pushing until the error is zero and the cylinder is pushing with a force that corresponds with the original command. Thus, any force variation induced in the system will likewise be automatically corrected.
The ultimate goal for many closed-loop hydraulic cylinder applications is twofold. First, to move into position, then to deliver a specific amount of hydraulic force to a work piece. The two operations must be done smoothly and seamlessly, though unfortunately, they are generally conducted at the expense of each other. Cylinder movement is performed prior to reaching the desired position at the work piece and is measured as displacement. Force is generated only once the work piece is met and is measured as pressure. Both operations are closed-loop controlled, though the operating characteristics for each are vastly different. To move the cylinder rapidly requires a high flow gain as compared to position error, meaning a relatively large valve. To move more slowly and precisely into position requires a low flow gain as compared to position error which requires a relatively smaller valve. Finally, building pressure once the cylinder has met the work piece requires extremely low flow gain as compared to pressure error signal which requires a very small valve and a dynamic system that is quite difficult to control.
Taking all of these requirements together has challenged the skilled artisan to achieve a hydraulic system which can rapidly move into position, accurately apply the correct pressure and also respond to a load. Since a hydraulic press is often a production machine, speed, part throughput, and tonnage capacity are paramount yet these are also in conflict with regards to developing a hydraulic system, and controller, which can achieve all of these goals while still responding to ever present budget constraints.
It is these conflicting control issues which have been circumvented with the present invention. Described herein is a hydraulic system, and control mechanism, which allows for rapid movement, accurate control and precision at the work piece.
It is an object of the present invention to provide an improved closed loop electrohydraulic system.
Particular features of the present invention include faster, more accurate, motion control by utilizing both position and force loop control.
These and other advantages, as will be realized, are provide in a hydraulic control system for controlling expansion and contraction of a cylinder. The cylinder comprises an extension chamber and a contraction chamber. The hydraulic control system comprises a first servovalve comprising a first pressure supply port, 100, and a first pressure discharge port, 101, and adapted to selectively communicate the expansion chamber with the first pressure supply port and with the first pressure discharge port. A second servovalve, comprising a second pressure supply port, 102, and a second pressure discharge port, 103, is adapted to selectively communicate the contraction chamber with the second pressure supply port and with the second pressure discharge port. A four way servovalve, comprising a third pressure supply port, 104, a third pressure discharge port, 105, a fourth pressure supply port, 106, and a fourth pressure discharge port, 107, is adapted to selectively communicate the expansion chamber with the third pressure supply port and the third pressure discharge port and to selectively communicate the contraction chamber with the fourth pressure supply port and with the fourth pressure discharge port. A feedback device is provided which is capable of measuring at least one property of the cylinder and providing a signal proportional to the property. A controller is provided which is capable of receiving the signal and comparing the property to a desired property to determine a difference and activating at least one of the first servovalve, the second servovalve or the four way servovalve to decrease the difference.